Analysis of cultured keratinocytes from a transgenic mouse model of psoriasis: effects of suprabasal integrin expression on keratinocyte adhesion, proliferation and terminal differentiation

Development of pluripotent stem cell-derived epidermal organoids that generate effective extracellular vesicles in skin regeneration

Cellular skin substitutes such as epidermal constructs have been developed for various applications, including wound healing and skin regeneration. These cellular models are mostly derived from primary cells such as keratinocytes and fibroblasts in a two-dimensional (2D) state, and further development of three-dimensional (3D) cultured organoids is needed to provide insight into the in vivo epidermal phenotype and physiology. Here, we report the development of epidermal organoids (EpiOs) generated from induced pluripotent stem cells (iPSCs) as a novel epidermal construct and its application as a source of secreted biomolecules recovered by extracellular vesicles (EVs) that can be utilized for cell-free therapy of regenerative medicine. Differentiated iPSC-derived epidermal organoids (iEpiOs) are easily cultured and expanded through multiple organoid passages, while retaining molecular and functional features similar to in vivo epidermis. These mature iEpiOs contain epidermal stem cell populations and retain the ability to further differentiate into other skin compartment lineages, such as hair follicle stem cells. By closely recapitulating the epidermal structure, iEpiOs are expected to provide a more relevant microenvironment to influence cellular processes and therapeutic response. Indeed, iEpiOs can generate high-performance EVs containing high levels of the angiogenic growth factor VEGF and miRNAs predicted to regulate cellular processes such as proliferation, migration, differentiation, and angiogenesis. These EVs contribute to target cell proliferation, migration, and angiogenesis, providing a promising therapeutic tool for in vivo wound healing. Overall, the newly developed iEpiOs strategy as an organoid-based approach provides a powerful model for studying basic and translational skin research and may also lead to future therapeutic applications using iEpiOs-secreted EVs.

Hydrogen peroxide production by epidermal dual oxidase 1 regulates nociceptive sensory signals

Keratinocytes of the mammalian skin provide not only mechanical protection for the tissues, but also transmit mechanical, chemical, and thermal stimuli from the external environment to the sensory nerve terminals. Sensory nerve fibers penetrate the epidermal basement membrane and function in the tight intercellular space among keratinocytes. Here we show that epidermal keratinocytes produce hydrogen peroxide upon the activation of the NADPH oxidase dual oxidase 1 (DUOX1). This enzyme can be activated by increasing cytosolic calcium levels. Using DUOX1 knockout animals as a model system we found an increased sensitivity towards certain noxious stimuli in DUOX1-deficient animals, which is not due to structural changes in the skin as evidenced by detailed immunohistochemical and electron-microscopic analysis of epidermal tissue. We show that DUOX1 is expressed in keratinocytes but not in the neural sensory pathway. The release of hydrogen peroxide by activated DUOX1 alters both the activity of neuronal TRPA1 and redox-sensitive potassium channels expressed in dorsal root ganglia primary sensory neurons. We describe hydrogen peroxide, produced by DUOX1 as a paracrine mediator of nociceptive signal transmission. Our results indicate that a novel, hitherto unknown redox mechanism modulates noxious sensory signals.

ITGA9 Inhibits Proliferation and Migration of Dermal Microvascular Endothelial Cells in Psoriasis

Background

Cell proliferation, migration, and angiogenesis are aberrant in psoriatic human dermal microvascular endothelial cells (HDMECs), resulting in abnormal endothelial function and microvascular dilation in psoriasis.

Objective

To explore the role of Integrin subunit alpha 9 (ITGA9) in proliferation and migration of dermal microvascular endothelial cells.

Methods

HDMECs were isolated from the skin of 6 psoriatic patients and 6 healthy controls. Expression levels of ITGA9 mRNA and protein were assessed with qRT-PCR and Western blot, respectively, while miqRT-PCR was used to determine expression levels of miR-146a-3p. Cell proliferation and migration were assessed in human microvascular endothelial cell line (HMEC-1), following overexpression of either ITGA9 or miR-146a-3p, or co-transfection with miR-146a-3p-mimic and pLVX - ITGA9. Cell viability was detected by Cell Counting Kit-8 assay and 5-ethynyl-2'-deoxyuridine (EdU) cell proliferation assay. Cell apoptosis was assessed, using annexin V-FITC/PI apoptosis detection kit, while cell migration was detected by wound healing and transwell assay.

Results

Expression levels of ITGA9 were significantly decreased in psoriatic HDMECs compared to normal controls. Moreover, expression levels of miR-146a-3p were higher in psoriatic HDMECs than in normal controls. Overexpression of miR-146a-3p lowered expression levels of ITGA9, accompanied by increased proliferation and migration of HMEC-1 in vitro. In contrast, overexpression of ITGA9 inhibited proliferation and migration of HMEC-1, while increasing expression levels of cdc42, ki67, focal adhesion kinase (FAK), c-Src tyrosine kinase (Src), RAC1 and RhoA.

Conclusion

ITGA9 can repress the proliferation and migration of HMEC-1, suggesting utility of ITGA9 as a potential therapeutic intervention for psoriasis.

Long-term expansion and differentiation of adult murine epidermal stem cells in 3D organoid cultures

Mammalian epidermal stem cells maintain homeostasis of the skin epidermis and contribute to its regeneration throughout adult life. While 2D mouse epidermal stem cell cultures have been established decades ago, a long-term, feeder cell- and serum-free culture system recapitulating murine epidermal architecture has not been available. Here we describe an epidermal organoid culture system that allows long-term, genetically stable expansion of adult epidermal stem cells. Our epidermal expansion media combines atypically high calcium concentrations, activation of cAMP, FGF, and R-spondin signaling with inhibition of bone morphogenetic protein (BMP) signaling. Organoids are established robustly from adult mouse skin and expand over at least 6 mo, while maintaining the basal-apical organization of the mouse interfollicular epidermis. The system represents a powerful tool to study epidermal homeostasis and disease in vitro.

Epidermal glucocorticoid and mineralocorticoid receptors act cooperatively to regulate epidermal development and counteract skin inflammation

Endogenous and synthetic glucocorticoids (GCs) regulate epidermal development and combat skin inflammatory diseases. GC actions can be mediated through the GC receptor (GR) and/or the mineralocorticoid receptor (MR), highly homologous ligand-activated transcription factors. While the role of GR as a potent anti-inflammatory mediator is well known, that of MR is not as clear, nor is whether these receptors cooperate or antagonize each other in the epidermis. To address this, we generated mice with epidermal-specific loss of both receptors (double knockout, DKO), and analyzed the phenotypical and functional consequences relative to single KOs or controls (CO). At birth, DKO epidermis displayed a phenotype of defective differentiation and inflammation, which was more severe than in either single KO, featuring neutrophil-containing infiltrates, and gene dysregulation characteristic of human psoriatic lesions. This phenotype resolved spontaneously. However, in adulthood, single or combined loss of GC receptors increased susceptibility to inflammation and hyperproliferation triggered by phorbol ester which, different to CO, was not effectively counteracted by GC treatment. Also, DKOs were more susceptible to imiquimod-induced psoriasis than CO showing severe defective epidermal differentiation and microabcesses while single KOs showed an intermediate response. Immortalized DKO keratinocytes featured increased proliferation kinetics and reduced cell size, a unique phenotype relative to single KO cells. The lack of GR and MR in keratinocytes, individual or combined, caused constitutive increases in p38 and ERK activities, which were partially reversed upon reinsertion of receptors into DKO cells. DKO keratinocytes also displayed significant increases in AP-1 and NF-κB transcriptional activities, which were partially rescued by ERK and p38 inhibition, respectively. Reinsertion of GR and MR in DKO keratinocytes resulted in physical and cooperative functional interactions that restored the transcriptional response to GCs. In conclusion, our data have revealed that epidermal GR and MR act cooperatively to regulate epidermal development and counteract skin inflammation.

Psoriasiform skin disease in transgenic pigs with high-copy ectopic expression of human integrins α2 and β1

Psoriasis is a complex human-specific disease characterized by perturbed keratinocyte proliferation and a pro-inflammatory environment in the skin. Porcine skin architecture and immunity are very similar to that in humans, rendering the pig a suitable animal model for studying the biology and treatment of psoriasis. Expression of integrins, which is normally confined to the basal layer of the epidermis, is maintained in suprabasal keratinocytes in psoriatic skin, modulating proliferation and differentiation as well as leukocyte infiltration. Here, we generated minipigs co-expressing integrins α2 and β1 in suprabasal epidermal layers. Integrin-transgenic minipigs born into the project displayed skin phenotypes that correlated with the number of inserted transgenes. Molecular analyses were in good concordance with histological observations of psoriatic hallmarks, including hypogranulosis and T-lymphocyte infiltration. These findings mark the first creation of minipigs with a psoriasiform phenotype resembling human psoriasis and demonstrate that integrin signaling plays a key role in psoriasis pathology.

Summary: A cloned porcine disease model to advance topical treatment in the debilitating skin disorder psoriasis.

A Method for the Immortalization of Newborn Mouse Skin Keratinocytes

Isolation and culture of mouse primary epidermal keratinocytes is a common technique that allows for easy genetic and environmental manipulation. However, due to their limited lifespan in culture, experiments utilizing primary keratinocytes require large numbers of animals, and are time consuming and expensive. To avoid these issues, we developed a method for the immortalization of primary mouse epidermal keratinocytes. Upon isolation of newborn epidermal keratinocytes according to established methods, the cells were cultured long-term in keratinocyte growth factor-containing medium. The cells senesced within a few weeks and eventually, small, slowly growing colonies emerged. After they regained confluency, the cells were passaged and slowly refilled the dish. With several rounds of subculture, the cells adapted to culture conditions, were easily subcultured, maintained normal morphology, and were apparently immortal. The immortalized cells retained the ability to differentiate with increased calcium concentrations, and were maintained to high passage numbers while maintaining a relatively stable karyotype. Analysis of multiple immortalized cell lines as well as primary keratinocyte cultures revealed increased numbers of chromosomes, especially in the primary keratinocytes, and chromosomal aberrations in most of the immortalized cultures and in the primary keratinocytes. Orthotopic grafting of immortalized keratinocytes together with fibroblasts onto nude mouse hosts produced skin while v-ras^Ha infection of the immortalized keratinocytes prior to grafting produced squamous cell carcinoma. In summary, this method of cell line generation allows for decreased use of animals, reduces the expense and time involved in research, and provides a useful model for cutaneous keratinocyte experimentation.

Epidermal Wnt/β-catenin signaling regulates adipocyte differentiation via secretion of adipogenic factors

It has long been recognized that the hair follicle growth cycle and oscillation in the thickness of the underlying adipocyte layer are synchronized. Although factors secreted by adipocytes are known to regulate the hair growth cycle, it is unclear whether the epidermis can regulate adipogenesis. We show that inhibition of epidermal Wnt/β-catenin signaling reduced adipocyte differentiation in developing and adult mouse dermis. Conversely, ectopic activation of epidermal Wnt signaling promoted adipocyte differentiation and hair growth. When the Wnt pathway was activated in the embryonic epidermis, there was a dramatic and premature increase in adipocytes in the absence of hair follicle formation, demonstrating that Wnt activation, rather than mature hair follicles, is required for adipocyte generation. Epidermal and dermal gene expression profiling identified keratinocyte-derived adipogenic factors that are induced by β-catenin activation. Wnt/β-catenin signaling-dependent secreted factors from keratinocytes promoted adipocyte differentiation in vitro, and we identified ligands for the bone morphogenetic protein and insulin pathways as proadipogenic factors. Our results indicate epidermal Wnt/β-catenin as a critical initiator of a signaling cascade that induces adipogenesis and highlight the role of epidermal Wnt signaling in synchronizing adipocyte differentiation with the hair growth cycle.

Development of transgenic cloned pig models of skin inflammation by DNA transposon-directed ectopic expression of human β1 and α2 integrin

Integrins constitute a superfamily of transmembrane signaling receptors that play pivotal roles in cutaneous homeostasis by modulating cell growth and differentiation as well as inflammatory responses in the skin. Subrabasal expression of integrins α2 and/or β1 entails hyperproliferation and aberrant differentiation of keratinocytes and leads to dermal and epidermal influx of activated T-cells. The anatomical and physiological similarities between porcine and human skin make the pig a suitable model for human skin diseases. In efforts to generate a porcine model of cutaneous inflammation, we employed the Sleeping Beauty DNA transposon system for production of transgenic cloned Göttingen minipigs expressing human β1 or α2 integrin under the control of a promoter specific for subrabasal keratinocytes. Using pools of transgenic donor fibroblasts, cloning by somatic cell nuclear transfer was utilized to produce reconstructed embryos that were subsequently transferred to surrogate sows. The resulting pigs were all transgenic and harbored from one to six transgene integrants. Molecular analyses on skin biopsies and cultured keratinocytes showed ectopic expression of the human integrins and localization within the keratinocyte plasma membrane. Markers of perturbed skin homeostasis, including activation of the MAPK pathway, increased expression of the pro-inflammatory cytokine IL-1α, and enhanced expression of the transcription factor c-Fos, were identified in keratinocytes from β1 and α2 integrin-transgenic minipigs, suggesting the induction of a chronic inflammatory phenotype in the skin. Notably, cellular dysregulation obtained by overexpression of either β1 or α2 integrin occurred through different cellular signaling pathways. Our findings mark the creation of the first cloned pig models with molecular markers of skin inflammation. Despite the absence of an overt psoriatic phenotype, these animals may possess increased susceptibility to severe skin damage-induced inflammation and should be of great potential in studies aiming at the development and refinement of topical therapies for cutaneous inflammation including psoriasis.

Epidermal α6β4 integrin stimulates the influx of immunosuppressive cells during skin tumor promotion

BACKGROUND

Induction of α6β4 integrin in the differentiated epidermal cell layers in skin is a hallmark of human cutaneous squamous cell carcinoma (SCC) pathogenesis and stimulates chemically induced SCC formation in Invα6β4 transgenic mice, which exhibit persistent expression of α6β4 in the suprabasal epidermal layers. However, the molecular basis for the support of SCC development by suprabasal α6β4 is not fully understood.

OBJECTIVE

We examined the relevance for suprabasal α6β4 expression in the epidermis for the recruitment of immunosuppressive leukocytes during the early stages of tumor promotion.

METHODS

In this study, we made use of the Invα6β4 transgenic mouse model, which exhibits expression of α6β4 integrin in the suprabasal layers of the epidermis driven by the involucrin promoter. First, we examined protein lysates from Invα6β4 transgenic skin using a pro-inflammatory cytokine array panel. Next, we immunofluorescence labeling of murine skin sections was employed to immunophenotype tumor promoter-treated Invα6β4 transgenic skin. Finally, a macrophage colony stimulating factor (M-CSF) neutralizing antibody strategy was administered to resolve Invα6β4 transgenic skin inflammation.

RESULTS

Employing the Invα6β4 transgenic mouse model, we show that suprabasal α6β4 integrin expression selectively alters the profile of secreted pro-inflammatory molecules by epidermal cells, in particular CXCL5 and M-CSF, in response to acute tumor promoter treatment. The induction of CXCL5 and M-CSF in Invα6β4 transgenic epidermis was shortly followed by an exacerbated influx of CD200R(+) myeloid-derived suppressor cells (MDSCs), which co-expressed the M-CSF receptor, and FoxP3(+) Treg cells compared to wild-type mice. As a result, the levels of activated CD4(+) T lymphocytes were dramatically diminished in Invα6β4 transgenic compared to wild-type skin, whereas similar levels of lymphocyte activation were observed in the peripheral blood. Finally, 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced CD200R(+) infiltrative cells and epidermal proliferation were suppressed in Invα6β4 mice treated with M-CSF neutralizing antibodies.

CONCLUSIONS

We conclude that aberrant expression of α6β4 integrin in post-mitotic epidermal keratinocytes stimulates a pro-tumorigenic skin microenvironment by augmenting the influx of immunosuppressive granular cells during tumor promotion.

Upregulation of CD26 expression in epithelial cells and stromal cells during wound-induced skin tumour formation

We have previously described InvEE transgenic mice in which non-dividing, differentiating epidermal cells express oncogenically activated MAPK kinase 1 (MEK1). Skin wounding triggers tumour formation in InvEE mice via a mechanism that involves epidermal release of IL-1α and attraction of a pro-tumorigenic inflammatory infiltrate. To look for potential effects on the underlying connective tissue, we screened InvEE and wild-type epidermis for differential expression of cytokines and immune modulators. We identified a single protein, CD26 (dipeptidyl peptidase-4). CD26 serum levels were not increased in InvEE mice. In contrast, CD26 was upregulated in keratinocytes expressing mutant MEK1 and in the epithelial compartment of InvEE tumours, where it accumulated at cell-cell borders. CD26 expression was increased in dermal fibroblasts following skin wounding but was downregulated in tumour stroma. CD26 activity was stimulated by calcium-induced intercellular adhesion in keratinocytes, suggesting that the upregulation of CD26 in InvEE epidermis is due to expansion of the differentiated cell layers. IL-1α treatment of dermal fibroblasts stimulated CD26 activity, and therefore epidermal IL-1α release may contribute to the upregulation of CD26 expression in wounded dermis. Pharmacological blockade of CD26, via Sitagliptin, reduced growth of InvEE tumours, while combined inhibition of IL-1α and CD26 delayed tumour onset and reduced tumour incidence. Our results demonstrate that inappropriate activation of MEK1 in the epidermis leads to changes in dermal fibroblasts that, like the skin inflammatory infiltrate, contribute to tumour formation.

Assaying proliferation and differentiation capacity of stem cells using disaggregated adult mouse epidermis

In this protocol, we describe how to isolate keratinocytes from adult mouse epidermis, fractionate them into different sub-populations on the basis of cell surface markers and examine their function in an in vivo skin reconstitution assay with disaggregated neonatal dermal cells. We also describe how the isolated keratinocytes can be subjected to clonal analysis in vitro and in vivo and how to enrich for hair follicle-inducing dermal papilla cells in the dermal preparation. Using these approaches, it is possible to compare the capacity of different populations of adult epidermal stem cells to proliferate and to generate progeny that differentiate along the different epidermal lineages. Isolating, fractionating and grafting cells for the skin reconstitution assay is normally spread over 2 d. Clonal growth in culture is assessed after 14 d, while evaluation of the grafts is carried out after 4-5 weeks.

CYLD negatively regulates cell-cycle progression by inactivating HDAC6 and increasing the levels of acetylated tubulin

CYLD is a tumour-suppressor gene that is mutated in a benign skin tumour syndrome called cylindromatosis. The CYLD gene product is a deubiquitinating enzyme that was shown to regulate cell proliferation, cell survival and inflammatory responses, mainly through inhibiting NF-kappaB signalling. Here we show that CYLD controls cell growth and division at the G(1)/S-phase as well as cytokinesis by associating with alpha-tubulin and microtubules through its CAP-Gly domains. Translocation of activated CYLD to the perinuclear region of the cell is achieved by an inhibitory interaction of CYLD with histone deacetylase-6 (HDAC6) leading to an increase in the levels of acetylated alpha-tubulin around the nucleus. This facilitates the interaction of CYLD with Bcl-3, leading to a significant delay in the G(1)-to-S-phase transition. Finally, CYLD also interacts with HDAC6 in the midbody where it regulates the rate of cytokinesis in a deubiquitinase-independent manner. Altogether these results identify a mechanism by which CYLD regulates cell proliferation at distinct cell-cycle phases.

Induced keratinocyte hyper-proliferation in alpha2beta1 integrin transgenic mice results in systemic immune cell activation

alpha2beta1 integrins are normally confined to the proliferating basal layers of the epidermis. However, during wound healing and in psoriasis, these integrins are expressed on keratinocytes in suprabasal layers correlating with a less differentiated phenotype. Transgenic mice expressing alpha2beta1 integrins under the involucrine promoter have previously been demonstrated, to various degrees, spontaneously develop a skin disorder resembling psoriasis. Herein, we show that a mild epidermal wounding induces a uniform acanthosis together with an influx of immune cells. The disease initiates as a normal wound healing process and is completely restored in wildtype mice by day 14. However, in the integrin transgenic mice a chronic inflammation develops, a process that can be compared to the Koebner phenomenon in psoriatic patients. In this study, we have followed the integrin transgenic mice for five weeks, where substantial keratinocyte hyper-proliferation, inflammatory infiltration and high cytokine levels within the skin can still be observed. In addition, draining lymph nodes were dramatically increased in size and contained highly activated T cells, as well as APCs secreting large amounts of pro-inflammatory cytokines. Furthermore, the systemic immune response was affected with increased spleen size, elevated cytokine levels in the serum and altered lymphocyte trafficking patterns, very much resembling what is seen in psoriasis patients. Finally, CD4(+) T cell depletion was not able to affect the onset or progression of skin inflammation. This suggests that altered keratinocyte differentiation and proliferation can drive a skin inflammation and cause chronic immune cell activation both at a local and systemic level.

The caspase-cleaved form of LYN mediates a psoriasis-like inflammatory syndrome in mice

We showed previously that Lyn is a substrate for caspases, a family of cysteine proteases, involved in the regulation of apoptosis and inflammation. Here, we report that expression of the caspase-cleaved form of Lyn (LynDeltaN), in mice, mediates a chronic inflammatory syndrome resembling human psoriasis. Genetic ablation of TNF receptor 1 in a LynDeltaN background rescues a normal phenotype, indicating that LynDeltaN mice phenotype is TNF-alpha-dependent. The predominant role of T cells in the disease occurring in LynDeltaN mice was highlighted by the distinct improvement of LynDeltaN mice phenotype in a Rag1-deficient background. Using pan-genomic profiling, we also established that LynDeltaN mice show an increased expression of STAT-3 and inhibitory members of the NFkappaB pathway. Accordingly, LynDeltaN alters NFkappaB activity underlying a link between inhibition of NFkappaB and LynDeltaN mice phenotype. Finally, analysis of Lyn expression in human skin biopsies of psoriatic patients led to the detection of Lyn cleavage product whose expression correlates with the activation of caspase 1. Our data identify a new role for Lyn as a regulator of psoriasis through its cleavage by caspases.

Beta1 integrin-mediated adhesion signalling is essential for epidermal progenitor cell expansion

BACKGROUND

There is a major discrepancy between the in vitro and in vivo results regarding the role of beta1 integrins in the maintenance of epidermal stem/progenitor cells. Studies of mice with skin-specific ablation of beta1 integrins suggested that epidermis can form and be maintained in their absence, while in vitro data have shown a fundamental role for these adhesion receptors in stem/progenitor cell expansion and differentiation.

METHODOLOGY/PRINCIPAL FINDINGS

To elucidate this discrepancy we generated hypomorphic mice expressing reduced beta1 integrin levels on keratinocytes that developed similar, but less severe defects than mice with beta1-deficient keratinocytes. Surprisingly we found that upon aging these abnormalities attenuated due to a rapid expansion of cells, which escaped or compensated for the down-regulation of beta1 integrin expression. A similar phenomenon was observed in aged mice with a complete, skin-specific ablation of the beta1 integrin gene, where cells that escaped Cre-mediated recombination repopulated the mutant skin in a very short time period. The expansion of beta1 integrin expressing keratinocytes was even further accelerated in situations of increased keratinocyte proliferation such as wound healing.

CONCLUSIONS/SIGNIFICANCE

These data demonstrate that expression of beta1 integrins is critically important for the expansion of epidermal progenitor cells to maintain epidermal homeostasis.

Loss of Kindlin-1 causes skin atrophy and lethal neonatal intestinal epithelial dysfunction

Kindler Syndrome (KS), characterized by transient skin blistering followed by abnormal pigmentation, skin atrophy, and skin cancer, is caused by mutations in the FERMT1 gene. Although a few KS patients have been reported to also develop ulcerative colitis (UC), a causal link to the FERMT1 gene mutation is unknown. The FERMT1 gene product belongs to a family of focal adhesion proteins (Kindlin-1, -2, -3) that bind several β integrin cytoplasmic domains. Here, we show that deleting Kindlin-1 in mice gives rise to skin atrophy and an intestinal epithelial dysfunction with similarities to human UC. This intestinal dysfunction results in perinatal lethality and is triggered by defective intestinal epithelial cell integrin activation, leading to detachment of this barrier followed by a destructive inflammatory response.

Mutations in FERMT1, coding for the Kindlin-1 protein, cause Kindler Syndrome in humans, characterized by skin blistering, atrophy, and cancer. Recent reports showed that some Kindler Syndrome patients additionally suffer from ulcerative colitis. However, it is unknown whether this is caused by loss of Kindlin-1 or by unrelated abnormalities such as infections or additional mutations. We ablated the Fermt1 gene in mice to directly analyze the pathological consequences and the molecular mode of action of Kindlin-1. Kindlin-1–deficient mice develop a severe epidermal atrophy, but lack blisters. All mutant mice die shortly after birth from a dramatic, shear force-induced detachment of intestinal epithelial cells followed by a profound inflammation and organ destruction. The intestinal phenotype is very similar to, although more severe than, the one observed in Kindler Syndrome patients. In vitro studies revealed that impaired integrin activation, and thus impaired adhesion, to the extracellular matrix of the intestinal wall causes intestinal epithelial cell detachment. Therefore, we demonstrate that intestinal epithelial cells require adhesive function of integrins to resist the shear force applied by the stool. Furthermore, we provide evidence that the colitis associated with Kindler Syndrome is caused by a dysfunction of Kindlin-1 rather than by a Kindlin-1–independent event.

Epidermal expression of the truncated prelamin A causing Hutchinson-Gilford progeria syndrome: effects on keratinocytes, hair and skin

Hutchinson-Gilford progeria syndrome (HGPS) is an accelerated aging disorder caused by point mutation in LMNA encoding A-type nuclear lamins. The mutations in LMNA activate a cryptic splice donor site, resulting in expression of a truncated, prenylated prelamin A called progerin. Expression of progerin leads to alterations in nuclear morphology, which may underlie pathology in HGPS. We generated transgenic mice expressing progerin in epidermis under control of a keratin 14 promoter. The mice had severe abnormalities in morphology of skin keratinocyte nuclei, including nuclear envelope lobulation and decreased nuclear circularity not present in transgenic mice expressing wild-type human lamin A. Primary keratinocytes isolated from these mice had a higher frequency of nuclei with abnormal shape compared to those from transgenic mice expressing wild-type human lamin A. Treatment with a farnesyltransferase inhibitor significantly improved nuclear shape abnormalities and induced the formation of intranuclear foci in the primary keratinocytes expressing progerin. Similarly, spontaneous immortalization of progerin-expressing cultured keratinocytes selected for cells with normal nuclear morphology. Despite morphological alterations in keratinocyte nuclei, mice expressing progerin in epidermis had normal hair grown and wound healing. Hair and skin thickness were normal even after crossing to Lmna null mice to reduce or eliminate expression of normal A-type lamins. Although progerin induces significant alterations in keratinocyte nuclear morphology that are reversed by inhibition of farnesyltransferasae, epidermal expression does not lead to alopecia or other skin abnormalities typically seen in human subjects with HGPS.

Role of the Notch ligand Delta1 in embryonic and adult mouse epidermis

The Notch ligand Delta1 (Dll1) is expressed in human interfollicular epidermis (IFE) and regulates differentiation and adhesion of cultured human keratinocytes. However, the consequences of deleting Dll1 in mouse epidermis have not been examined. Here, we report that in embryonic mouse skin Dll1 is expressed by patches of keratinocytes in the basal layer of the IFE and in the dermal papilla and hair bulb. In a Dll1 hypomorph mutant that survives until birth, hair follicles formed normally but proliferation and thickness of the IFE were increased. Deletion of Dll1 using Cre recombinase expressed under the control of the keratin-5 (K5) promoter resulted in a delay in the first postnatal anagen, but subsequent hair cycles were normal. As in the hypomorph, IFE proliferation was stimulated and expression of K10 and K17 was disturbed. Older mice developed tumors with elements of IFE differentiation. Keratinocytes cultured from K5Cre x Dll1(flox/flox) epidermis showed a transient increase in proliferation, with a subsequent decrease in integrin expression and increased terminal differentiation. These results demonstrate that Dll1 contributes to the control of proliferation and differentiation in IFE, whereas Jagged1 regulates hair follicle differentiation.

Integrin-linked kinase is required for epidermal and hair follicle morphogenesis

Integrin-linked kinase (ILK) links integrins to the actin cytoskeleton and is believed to phosphorylate several target proteins. We report that a keratinocyte-restricted deletion of the ILK gene leads to epidermal defects and hair loss. ILK-deficient epidermal keratinocytes exhibited a pronounced integrin-mediated adhesion defect leading to epidermal detachment and blister formation, disruption of the epidermal–dermal basement membrane, and the translocation of proliferating, integrin-expressing keratinocytes to suprabasal epidermal cell layers.

The mutant hair follicles were capable of producing hair shaft and inner root sheath cells and contained stem cells and generated proliferating progenitor cells, which were impaired in their downward migration and hence accumulated in the outer root sheath and failed to replenish the hair matrix. In vitro studies with primary ILK-deficient keratinocytes attributed the migration defect to a reduced migration velocity and an impaired stabilization of the leading-edge lamellipodia, which compromised directional and persistent migration. We conclude that ILK plays important roles for epidermis and hair follicle morphogenesis by modulating integrin-mediated adhesion, actin reorganization, and plasma membrane dynamics in keratinocytes.

Analysis of integrin functions in peri-implantation embryos, hematopoietic system, and skin

Integrins mediate cell adhesion, permit traction forces important for cell migration, and cross-talk with growth factor receptors to regulate cell proliferation, cell survival, and cell differentiation. The plethora of functions explains their central role for development and disease. The progress in mouse genetics and the ease with which the mouse genome can be manipulated enormously contributed to our understanding of how integrins exert their functions at the molecular level. In the present chapter, we describe tests that are routinely used in our laboratory to investigate embryos, organs, and cells (peri-implantation embryos, hematopoietic system, epidermis, and hair follicles) that lack the expression of integrins or integrin-associated proteins.

Rac1 is crucial for hair follicle integrity but is not essential for maintenance of the epidermis

Rac1 is a small GTPase that regulates the actin cytoskeleton but also other cellular processes. To investigate the function of Rac1 in skin, we generated mice with a keratinocyte-restricted deletion of the rac1 gene. Rac1-deficient mice lost nearly all of their hair within a few weeks after birth. The nonpermanent part of mutant hair follicles developed constrictions; lost expression of hair follicle-specific keratins, E-cadherin, and alpha6 integrin; and was eventually removed by macrophages. The permanent part of hair follicles and the sebaceous glands were maintained, but no regrowth of full-length hair follicles was observed. In the skin of mutant mice, epidermal keratinocytes showed normal differentiation, proliferation, cell-cell contacts, and basement membrane deposition, demonstrating no obvious defects of Rac1-deficient epidermis in vivo. In vitro, Rac1-null keratinocytes displayed a strong spreading defect and slightly impaired adhesion. These data show that Rac1 plays an important role in sustaining the integrity of the lower part of hair follicles but not in maintenance of the epidermis.

Genetic analysis of beta1 integrin "activation motifs" in mice

Akey feature of integrins is their ability to regulate the affinity for ligands, a process termed integrin activation. The final step in integrin activation is talin binding to the NPXY motif of the integrin beta cytoplasmic domains. Talin binding disrupts the salt bridge between the alpha/beta tails, leading to tail separation and integrin activation. We analyzed mice in which we mutated the tyrosines of the beta1 tail and the membrane-proximal aspartic acid required for the salt bridge. Tyrosine-to-alanine substitutions abolished beta1 integrin functions and led to a beta1 integrin-null phenotype in vivo. Surprisingly, neither the substitution of the tyrosines with phenylalanine nor the aspartic acid with alanine resulted in an obvious defect. These data suggest that the NPXY motifs of the beta1 integrin tail are essential for beta1 integrin function, whereas tyrosine phosphorylation and the membrane-proximal salt bridge between alpha and beta1 tails have no apparent function under physiological conditions in vivo.

Cyld inhibits tumor cell proliferation by blocking Bcl-3-dependent NF-kappaB signaling

Mutations in the CYLD gene cause tumors of hair-follicle keratinocytes. The CYLD gene encodes a deubiquitinase that removes lysine 63-linked ubiquitin chains from TRAF2 and inhibits p65/p50 NF-kappaB activation. Here we show that mice lacking Cyld are highly susceptible to chemically induced skin tumors. Cyld-/- tumors and keratinocytes treated with 12-O-tetradecanoylphorbol-13 acetate (TPA) or UV light are hyperproliferative and have elevated cyclin D1 levels. The cyclin D1 elevation is caused not by increased p65/p50 action but rather by increased nuclear activity of Bcl-3-associated NF-kappaB p50 and p52. In Cyld+/+ keratinocytes, TPA or UV light triggers the translocation of Cyld from the cytoplasm to the perinuclear region, where Cyld binds and deubiquitinates Bcl-3, thereby preventing nuclear accumulation of Bcl-3 and p50/Bcl-3- or p52/Bcl-3-dependent proliferation. These data indicate that, depending on the external signals, Cyld can negatively regulate different NF-kappaB pathways; inactivation of TRAF2 controls survival and inflammation, while inhibition of Bcl-3 controls proliferation and tumor growth.

The cell-surface marker MTS24 identifies a novel population of follicular keratinocytes with characteristics of progenitor cells

We describe a novel murine progenitor cell population localised to a previously uncharacterised region between sebaceous glands and the hair follicle bulge, defined by its reactivity to the thymic epithelial progenitor cell marker MTS24. MTS24 labels a membrane-bound antigen present during the early stages of hair follicle development and in adult mice. MTS24 co-localises with expression of alpha6-integrin and keratin 14, indicating that these cells include basal keratinocytes. This novel population does not express the bulge-specific stem cell markers CD34 or keratin 15, and is infrequently BrdU label retaining. MTS24-positive and -negative keratinocyte populations were isolated by flow cytometry and assessed for colony-forming efficiency. MTS24-positive keratinocytes exhibited a two-fold increase in colony formation and colony size compared to MTS24-negative basal keratinocytes. In addition, both the MTS24-positive and CD34-positive subpopulations were capable of producing secondary colonies after serial passage of individual cell clones. Finally, gene expression profiles of MTS24 and CD34 subpopulations were compared. These results showed that the overall gene expression profile of MTS24-positive cells resembles the pattern previously reported in bulge stem cells. Taken together, these data suggest that the cell-surface marker MTS24 identifies a new reservoir of hair follicle keratinocytes with a proliferative capacity and gene expression profile suggestive of progenitor or stem cells.

Cdc42 controls progenitor cell differentiation and beta-catenin turnover in skin

Differentiation of skin stem cells into hair follicles (HFs) requires the inhibition of beta-catenin degradation, which is controlled by a complex containing axin and the protein kinase GSK3beta. Using conditional gene targeting in mice, we show now that the small GTPase Cdc42 is crucial for differentiation of skin progenitor cells into HF lineage and that it regulates the turnover of beta-catenin. In the absence of Cdc42, degradation of beta-catenin was increased corresponding to a decreased phosphorylation of GSK3beta at Ser 9 and an increased phosphorylation of axin, which is known to be required for binding of beta-catenin to the degradation machinery. Cdc42-mediated regulation of beta-catenin turnover was completely dependent on PKCzeta, which associated with Cdc42, Par6, and Par3. These data suggest that Cdc42 regulation of beta-catenin turnover is important for terminal differentiation of HF progenitor cells in vivo.

New roles for integrins in squamous-cell carcinoma

Although integrins are known to mediate invasion and metastasis, recent studies reveal new ways in which they contribute to squamous-cell carcinoma. Integrin mutation or upregulation can expand the tumour stem-cell compartment by inhibiting differentiation or apoptosis. Integrins that are expressed by differentiated cells can stimulate or inhibit the proliferation of neighbouring tumour stem cells. These findings provide a mechanistic basis for the well-established links between altered integrin expression and tumour prognosis.

Beta-catenin and Hedgehog signal strength can specify number and location of hair follicles in adult epidermis without recruitment of bulge stem cells

Using K14deltaNbeta-cateninER transgenic mice, we show that short-term, low-level beta-catenin activation stimulates de novo hair follicle formation from sebaceous glands and interfollicular epidermis, while only sustained, high-level activation induces new follicles from preexisting follicles. The Hedgehog pathway is upregulated by beta-catenin activation, and inhibition of Hedgehog signaling converts the low beta-catenin phenotype to wild-type epidermis and the high phenotype to low. beta-catenin-induced follicles contain clonogenic keratinocytes that express bulge markers; the follicles induce dermal papillae and provide a niche for melanocytes, and they undergo 4OHT-dependent cycles of growth and regression. New follicles induced in interfollicular epidermis are derived from that cellular compartment and not through bulge stem cell migration or division. These results demonstrate the remarkable capacity of adult epidermis to be reprogrammed by titrating beta-catenin and Hedgehog signal strength and establish that cells from interfollicular epidermis can acquire certain characteristics of bulge stem cells.

Endogenous Myc controls mammalian epidermal cell size, hyperproliferation, endoreplication and stem cell amplification

The transcription factor Myc (c-Myc) plays an important role in cell growth and cell death, yet its physiological function remains unclear. Ectopic activation of Myc has been recently suggested to regulate cell mass, and Drosophila dmyc controls cellular growth and size independently of cell division. By contrast, it has been proposed that in mammals Myc controls cell division and cell number. To gain insights into this debate we have specifically knocked out Myc in epidermis. Myc epidermal knockout mice are viable and their keratinocytes continue to cycle, but they display severe skin defects. The skin is tight and fragile, tears off in areas of mechanical friction and displays impaired wound healing. Steady-state epidermis is thinner, with loss of the proliferative compartment and premature differentiation. Remarkably, keratinocyte cell size, growth and endoreplication are reduced, and stem cell amplification is compromised. The results provide new and direct evidence for a role for endogenous Myc in cellular growth that is required for hyperproliferative cycles and tissue homeostasis.

Expression of activated MEK1 in differentiating epidermal cells is sufficient to generate hyperproliferative and inflammatory skin lesions

Epidermal activation of Erk MAPK is observed in human psoriatic lesions and in a mouse model of psoriasis in which beta1 integrins are expressed in the suprabasal epidermal layers. Constitutive activation of the upstream kinase MEK1 causes hyperproliferation and perturbed differentiation of human keratinocytes in culture. It is not known, however, whether Erk activation in differentiating keratinocytes is sufficient to trigger hyperproliferation of basal keratinocytes and a skin inflammatory infiltrate. To investigate this, we expressed constitutively active MEK1 in the suprabasal epidermal layers of transgenic mice. Proliferation in the epidermal basal layer was stimulated and epidermal terminal differentiation was perturbed. Some older mice also developed papillomas. There was a large increase in T lymphocytes, dendritic cells, and neutrophils in the skin. The effects of suprabasal MEK1 on basal keratinocytes and leukocytes, cells that were transgene negative, suggested that MEK1 activity might stimulate cytokine release. Transgenic keratinocytes expressed elevated IL-1alpha and crossing the mice with mice overexpressing the IL-1 receptor in the epidermal basal layer led to exacerbated hyperproliferation and inflammation. These data suggest that activation of MEK1 downstream of beta1 integrins plays an important role in epidermal hyperproliferation and skin inflammation.

Suprabasal alpha6beta4 integrin expression in epidermis results in enhanced tumourigenesis and disruption of TGFbeta signalling

Inappropriate alpha6beta4 integrin expression correlates with a high risk of tumour progression in stratified squamous epithelia. Targeted expression of alpha6beta4 in the suprabasal layers of transgenic mouse epidermis dramatically increased the frequency of papillomas, carcinomas and metastases induced by chemical carcinogenesis, independent of the beta4 cytoplasmic domain. Suprabasal alpha6beta4 also perturbed transforming growth factor beta (TGFbeta) signalling as demonstrated by decreased nuclear Smad2 in transgenic epidermis and tumours. In cultured keratinocytes, suprabasal alpha6beta4 relieved TGFbeta-mediated growth inhibition and blocked nuclear translocation of activated Smad2/3. Responsiveness to TGFbeta could be restored by inhibiting cadherin-mediated cell-cell adhesion or phosphoinositide 3-kinase (PI3-K) activity, but not by inhibiting mitogen-activated protein kinase (MAPK) activity. These data suggest that suprabasal alpha6beta4 promotes tumourigenesis by preventing TGFbeta from suppressing clonal expansion of initiated cells in the epidermal basal layer.

Regulation of interleukin-1alpha expression by integrins and epidermal growth factor receptor in keratinocytes from a mouse model of inflammatory skin disease

Transgenic mice expressing beta1 integrins in the suprabasal epidermal layers have sporadic skin hyperproliferation and inflammation correlated with activation of extracellular signal-regulated kinase (Erk) mitogen-activated protein kinase and increased interleukin (IL)-1alpha production. We investigated the link between aberrant integrin expression, Erk activation, and expression of IL-1alpha. Transgenic keratinocytes had higher basal Erk activity and IL-1alpha levels than nontransgenic controls and were more sensitive to stimulation of Erk activity and IL-1alpha production by IL-1alpha, 12-O-tetradecanoylphorbol-13-acetate (TPA), epidermal growth factor (EGF), and serum. Inhibition of Erk in transgenic keratinocytes reduced basal IL-1alpha levels and the stimulation of IL-1alpha production by serum or phorbol ester, demonstrating that Erk could regulate IL-1alpha expression. TPA or IL-1alpha treatment resulted in rapid down-regulation of the EGF receptor in transgenic cells, indicative of transactivation. Inhibition of transactivation blocked basal and TPA or IL-1alpha induced Erk activation, but not IkappaBalpha degradation, and abolished increased IL-1alpha production in transgenic cells. In transgene-negative cells, constitutive activation of IL-1-dependent signaling by wild type or kinase-dead IRAK1 stimulated IL-1alpha production independent of Erk. We conclude that suprabasal integrin expression leads to Erk activation and increased IL-1alpha expression by potentiating activation of the EGF receptor. These results provide a mechanism by which aberrant integrin expression triggers epidermal hyperproliferation and inflammation.

Role of integrins in regulating epidermal adhesion, growth and differentiation

Mammalian epidermis is renewed throughout life by proliferation of a multipotential stem cell population and terminal differentiation of stem cell progeny. In recent years, extracellular matrix receptors of the integrin family have been identified as important regulators of epidermal homeostasis, influencing the balance between stem cell renewal and differentiation. Integrin expression is altered when the epidermis is damaged or diseased, and there is good evidence that specific integrins can contribute positively or negatively to pathogenesis. In this review I summarize what is known about the expression and function of epidermal integrins, and highlight the challenges for future research.

TNF-mediated inflammatory skin disease in mice with epidermis-specific deletion of IKK2

The I kappa B kinase (IKK), consisting of the IKK1 and IKK2 catalytic subunits and the NEMO (also known as IKK gamma) regulatory subunit, phosphorylates I kappa B proteins, targeting them for degradation and thus inducing activation of NF-kappa B (reviewed in refs 1, 2). IKK2 and NEMO are necessary for NF-kappa B activation through pro-inflammatory signals. IKK1 seems to be dispensable for this function but controls epidermal differentiation independently of NF-kappa B. Previous studies suggested that NF-kappa B has a function in the growth regulation of epidermal keratinocytes. Mice lacking RelB or I kappa B alpha, as well as both mice and humans with heterozygous NEMO mutations, develop skin lesions. However, the function of NF-kappa B in the epidermis remains unclear. Here we used Cre/loxP-mediated gene targeting to investigate the function of IKK2 specifically in epidermal keratinocytes. IKK2 deficiency inhibits NF-kappa B activation, but does not lead to cell-autonomous hyperproliferation or impaired differentiation of keratinocytes. Mice with epidermis-specific deletion of IKK2 develop a severe inflammatory skin disease, which is caused by a tumour necrosis factor-mediated, alpha beta T-cell-independent inflammatory response that develops in the skin shortly after birth. Our results suggest that the critical function of IKK2-mediated NF-kappa B activity in epidermal keratinocytes is to regulate mechanisms that maintain the immune homeostasis of the skin.

A crucial role of β1 integrins for keratinocyte migration in vitro and during cutaneous wound repair

Integrins are ubiquitous transmembrane receptors that play crucial roles in cell-cell and cell-matrix interactions. In this study, we have determined the effects of the loss of β1 integrins in keratinocytes in vitro and during cutaneous wound repair. Flow cytometry of cultured β1-deficient keratinocytes confirmed the absence of β1 integrins and showed downregulation of α6β4 but not of αv integrins. β1-null keratinocytes were characterised by poor adhesion to various substrates, by a reduced proliferation rate and by a strongly impaired migratory capacity. In vivo, the loss of β1 integrins in keratinocytes caused a severe defect in wound healing. β1-null keratinocytes showed impaired migration and were more densely packed in the hyperproliferative epithelium. Surprisingly, their proliferation rate was not reduced in early wounds and even increased in late wounds. The failure in re-epithelialisation resulted in a prolonged inflammatory response, leading to dramatic alterations in the expression of important wound-regulated genes. Ultimately, β1-deficient epidermis did cover the wound bed, but the epithelial architecture was abnormal. These findings demonstrate a crucial role of β1 integrins in keratinocyte migration and wound re-epithelialisation.

Movies available on-line

Basal keratinocytes from uninvolved psoriatic skin exhibit accelerated spreading and focal adhesion kinase responsiveness to fibronectin

We previously proposed that the keratinocyte hyperproliferative state in psoriatic skin results from a combination of T cell cytokine interaction with basal keratinocytes that exist in a primed state. We now provide evidence that basal keratinocytes from psoriatic uninvolved skin are in a preactivated state with regard to their interaction with fibronectin. Freshly isolated basal keratinocytes (K(1)/K(10)(-)) from non-lesional psoriatic skin demonstrated a significantly higher percentage of spreading cells 1 h after plating on fibronectin-coated plates than keratinocytes isolated from normal skin (p =0.0002). No differences were observed on collagen-laminin-coated plates, however. The keratinocyte spreading on fibronectin-coated plates involved alpha 5 beta 1 and alpha V beta 1 integrins. To address the potential signaling cascades that may respond to integrin changes in psoriatic keratinocytes, focal adhesion kinase changes were assessed. The percentage of keratinocytes from psoriatic uninvolved skin that exhibit positive focal adhesion kinase staining was significantly greater than the percentage from healthy volunteers after 1 h incubation on fibronectin (p =0.006). Additionally, focal adhesion kinase isolated from uninvolved psoriatic keratinocytes had a greater degree of tyrosine phosphorylation. Thus, the proliferative effect of fibronectin in combination with T cell lymphokines on psoriatic uninvolved basal keratinocyte progenitors may be due to abnormal in vivo integrin-driven focal adhesion kinase activity and downstream signaling.

A role for mitogen-activated protein kinase activation by integrins in the pathogenesis of psoriasis

In normal epidermis, beta1 integrin expression is confined to the basal layer, whereas in hyperproliferative epidermis, integrins are also expressed in the suprabasal layers. Transgenic mice in which integrins are expressed suprabasally via the involucrin promoter have a sporadic psoriatic phenotype; however, the mechanism by which integrins contribute to the pathogenesis of psoriasis is unknown. We observed activation of mitogen-activated protein kinase (MAPK) in basal and suprabasal keratinocytes of human and transgenic mouse psoriatic lesions and healing mouse skin wounds, correlating in each case with suprabasal integrin expression. Phenotypically normal human and transgenic mouse epidermis did not contain activated MAPK. Transgene-positive keratinocytes produced more IL-1alpha than controls did, and keratinocyte MAPK could be activated by ligation of suprabasal integrins or treatment with IL-1alpha. Constitutive activation of MAPK increased the growth rate of human keratinocytes and delayed the onset of terminal differentiation, recreating many of the histological features of psoriatic epidermis. We propose that activation of MAPK by integrins, either directly or through increased IL-1alpha production, is responsible for epidermal hyperproliferation in psoriasis and wound healing, and that the sporadic phenotype of the transgenic mice may reflect the complex mechanisms by which IL-1 release and responsiveness are controlled in skin.

Clues to epidermal cancer proneness revealed by reconstruction of DNA repair-deficient xeroderma pigmentosum skin in vitro

Sun exposure has been clearly implicated in premature skin aging and neoplastic development. These features are exacerbated in patients with xeroderma pigmentosum (XP), a hereditary disease, the biochemical hallmark of which is a severe deficiency in the nucleotide excision repair of UV-induced DNA lesions. To develop an organotypic model of DNA repair deficiency, we have cultured several strains of primary XP keratinocytes and XP fibroblasts from skin biopsies of XP patients. XP skin comprising both a full-thickness epidermis and a dermal equivalent was successfully reconstructed in vitro. Satisfactory features of stratification were obtained, but the expression of epidermal differentiation products, such as keratin K10 and loricrin, was delayed and reduced. In addition, the proliferation of XP keratinocytes was more rapid than that of normal keratinocytes. Moreover, increased deposition of cell attachment proteins, alpha-6 and beta-1 integrins, was observed in the basement membrane zone, and beta-1 integrin subunit, the expression of which is normally confined to basal keratinocytes, extended into several suprabasal cell layers. Most strikingly, the in vitro reconstructed XP skin displayed numerous proliferative epidermal invasions within dermal equivalents. Epidermal invasion and higher proliferation rate are reminiscent of early steps of neoplasia. Compared with normal skin, the DNA repair deficiency of in vitro reconstructed XP skin was documented by long-lasting persistence of UVB-induced DNA damage in all epidermal layers, including the basal layer from which carcinoma develops. The availability of in vitro reconstructed XP skin provides opportunities for research in the fields of photoaging, photocarcinogenesis, and tissue therapy.

Subcellular distribution of envoplakin and periplakin: insights into their role as precursors of the epidermal cornified envelope

Envoplakin and periplakin are two plakins that are precursors of the epidermal cornified envelope. We studied their distribution and interactions by transfection of primary human keratinocytes and other cells. Full-length periplakin localized to desmosomes, the interdesmosomal plasma membrane and intermediate filaments. Full length envoplakin also localized to desmosomes, but mainly accumulated in nuclear and cytoplasmic aggregates with associated intermediate filaments. The envoplakin rod domain was required for aggregation and the periplakin rod domain was necessary and sufficient to redistribute envoplakin to desmosomes and the cytoskeleton, confirming earlier predictions that the proteins can heterodimerize. The linker domain of each protein was required for intermediate filament association. Like the NH(2) terminus of desmoplakin, that of periplakin localized to desmosomes; however, in addition, the periplakin NH(2) terminus accumulated at cell surface microvilli in association with cortical actin. Endogenous periplakin was redistributed from microvilli when keratinocytes were treated with the actin disrupting drug Latrunculin B. We propose that whereas envoplakin and periplakin can localize independently to desmosomes, the distribution of envoplakin at the interdesmosomal plasma membrane depends on heterodimerization with periplakin and that the NH(2) terminus of periplakin therefore plays a key role in forming the scaffold on which the cornified envelope is assembled.

beta1B integrin subunit contains a double lysine motif that can cause accumulation within the endoplasmic reticulum

Human epidermal keratinocytes are one of the few cell types that express the beta1B splice variant of the beta1 integrin subunit. Although in transfection experiments beta1B acts as a dominant negative inhibitor of cell adhesion, we found that beta1B was expressed at very low levels in keratinocytes, both in vivo and in culture, and had a predominantly cytoplasmic distribution, concentrated within the endoplasmic reticulum. To examine why beta1B accumulated in the cytoplasm, we prepared chimeras between CD8alpha and the beta1A and beta1B integrin cytoplasmic domains. In transfected HeLa cells, both constructs reached the cell surface but the rate of maturation of the beta1B chimera was considerably retarded relative to beta1A. The beta1B cytoplasmic domain contains two lysine residues that resemble the double lysine motif characteristic of many proteins that are resident within the endoplasmic reticulum. Mutation of each lysine individually to serine had no effect on CD8beta1B maturation, but when both residues were mutated the rate of CD8beta1B maturation increased to that of CD8beta1A. Further analysis of beta1B function in keratinocytes must, therefore, take into account the low abundance of the isoform relative to beta1A and the potential for beta1B to accumulate in the endoplasmic reticulum.

Role of integrins in mouse eyelid development: studies in normal embryos and embryos in which there is a failure of eyelid fusion

Eyelid fusion normally occurs between E15.5 and E16.5 of mouse embryonic development and results from the migration of a population of periderm-derived epithelial cells over the corneal surface. Cell migration is known to depend on extracellular matrix receptors of the integrin family and to be regulated by growth factors. We were therefore interested that a failure of eyelid fusion has been reported in mice that are homozygous null for the transforming growth factor alpha (TGF-alpha) gene and in mice (invalpha5beta1) in which a transgenic alpha5beta1 integrin under the control of the involucrin promoter is misexpressed in differentiating keratinocytes. We examined expression of the alpha2beta1, alpha3beta1, alpha5beta1 and alpha6beta4 integrins during eyelid fusion in wild-type embryos and found selective upregulation of the alpha5beta1 integrin and its ligand, fibronectin, in the migrating eyelid tip cells. In TGF-alpha null embryos, the failure of eyelid fusion was correlated with a failure to upregulate the alpha5beta1 integrin and fibronectin in the tip cells. Using beta-galactosidase as a reporter gene in transgenic mice, we observed specific activity of the involucrin promoter in the eyelid tip cells. In invalpha5beta1 mice the transgenic human integrin was overexpressed not only in the tip cells but throughout the eyelid epidermis. In contrast, the endogenous, murine, alpha5beta1 integrin was only weakly expressed in the tip cells. We speculate that selective and coordinated expression of the alpha5beta1 integrin and fibronectin in eyelid tip cells is required for eyelid fusion and may be under the control of growth factors that include TGF-alpha.